Turbo Alternator Rotor bars arcing
Turbo Alternator Rotor bars arcing
(OP)
We've got a 38 year old Brush Electrical Turbo alternator with the following equipment.
Control & Protection
• GE MULTILIN SR489, differential protection system (87/2G 1200:1 Class X).
• AVR has 1250/5 CT on A1 - A2 winding, Earth fault protection is through a floating 600/1 Neutral CT
Excitation System
Pilot Exciter is a 90Volt 1.35kVA 190Hz shaft mounted
Main Exciter is a 90kW, 1220V 407A with a field exciter of 27V, 5.4A
Main Alternator
23.375 MVA, 18.7 MW, 1225A, 11 KV , pf 0.8
The problem is that the rotor bars have extensive arcing marks on the copper bar windings. Also the end retaining rings show marks of pitting.
Control & Protection
• GE MULTILIN SR489, differential protection system (87/2G 1200:1 Class X).
• AVR has 1250/5 CT on A1 - A2 winding, Earth fault protection is through a floating 600/1 Neutral CT
Excitation System
Pilot Exciter is a 90Volt 1.35kVA 190Hz shaft mounted
Main Exciter is a 90kW, 1220V 407A with a field exciter of 27V, 5.4A
Main Alternator
23.375 MVA, 18.7 MW, 1225A, 11 KV , pf 0.8
The problem is that the rotor bars have extensive arcing marks on the copper bar windings. Also the end retaining rings show marks of pitting.





RE: Turbo Alternator Rotor bars arcing
Pitted end rings sounds like a corrosion problem. Rings are normally a forging for strength, so the base metal is of good and uniform quality. I'm not sure what testing methods were in use at Loughborough 40 years ago but I suspect they would ge good enough to pick up any gross problem with the end ring material.
What maintenance tests have been performed? Have you carried out an RSO recently? How does it compare to ones from earlier in the machine's life?
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If we learn from our mistakes I'm getting a great education!
RE: Turbo Alternator Rotor bars arcing
The ring forgings were delivered with a forgemasters certificate and i think a separate piece from the forging for factory analysis.
RE: Turbo Alternator Rotor bars arcing
Muthu
www.edison.co.in
RE: Turbo Alternator Rotor bars arcing
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If we learn from our mistakes I'm getting a great education!
RE: Turbo Alternator Rotor bars arcing
Have the retaining rings (end caps) ever been changed from original 18Mn/5Cr metallurgy? If not the older metallurgy is highly subject to SCC and especially if you are in an environment where moisture and/or (and especially) salt can be present (industrial or marine) ((industrial in USA venacular connotes plants with industrial type gases or processes nearby as to opposed to British venacular where industrial means electric utility)). Could it be cracking that you are seeing that you are seeing on the retaining rings? I promise you that if your rings are 38 years old, that they are 18/5. Brush didn't even start to change until the mid to late '80's.
If they were replaced prior to that, they still may be 18/5.
The newer 18/8 metallurgy retaining rings are said to be immune to SCC cracking.
Google the term "retaining ring metallurgy" and do some heavy reading.
I raise the same question as raised earlier with respect to the arcing comment. Arcing from where to where? Rotor bar to end cap or bar to bar? Did you (do you) have a rotor earth fault detector? And if so, did it take you out?
What about the insulation over the end windings? (between the end windings and the retaining rings.) Is the integrity of that still good? If it hasn't been changed in 38 years, I'd be suspicious of that.
Is the information you have given about the rotor condition with retaining rings in place or removed.
If the latter, is there signs of fretting in the rotor tooth to end cap fit area?
rmw
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
As a starter for a machine that size:
Stator differential
Phase overcurrent
Underexcitation
Overexcitation
Voltage-restrained or voltage-controlled overcurrent
Negative phase sequence
Rotor earth fault
Over-fluxing (might be on the GSU transformer)
Over-voltage
Pole-slip
Loss of excitation
Probably a few I can't think of right now. There are plenty of good texts - have a look in the FAQs.
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If we learn from our mistakes I'm getting a great education!
RE: Turbo Alternator Rotor bars arcing
FAQ238-1287: What are good references for a Power Engineer?
There are several free items, including art and science of protective relaying
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(2B)+(2B)' ?
RE: Turbo Alternator Rotor bars arcing
Add reverse power to my previous list.
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If we learn from our mistakes I'm getting a great education!
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
If you don't understand what synchronous reactance is and how it is found, and don't understand what the nameplate power factor is telling you, there are probably a lot of other questions you should be asking but don't know that you don't know. Your machine will thank you for getting local assistance.
RE: Turbo Alternator Rotor bars arcing
If the PF drops below 0.8 the kW output should be reduced accordingly to avoid overcurrent on the stator winding.
Note, many diesel sets rated for prime power "can supply 10% overload power for 1 hour in 12 hours."*
That equates to a PF of about 0.73, for rated kW output "for 1 hour in 12 hours."
I would be very comfortable running a prime rated diesel set at full kW output and 0.73 power factor "for 1 hour in 12 hours."
How does this compare with the rating methods on the big sets, Scotty?
Bill
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"Why not the best?"
Jimmy Carter
RE: Turbo Alternator Rotor bars arcing
RE: Turbo Alternator Rotor bars arcing
I checked this out on the F G Wilson website before I posted to be sure I had it right.
A number of years ago F.G. Wilson was number three in the world as a diesel gen-set builder. They have since been bought by Caterpillar.
They often use the same machine for a prime rated set as for a standby set. The prime set may have some added engine accessories such as an oil cooler, and/or larger oil sump.
This is the set that I based my comments on:
h
You can see from the specs that this is the same generator end and the same diesel engine.
As a prime rated set it is capable of 200 kW and 250 KVA. Model number S250P3.
As a prime rated set it is capable of 220 kW and 275 KVA but no overload is permitted. Model number S275E3.
So this set may safely handle 275 KVA. As a prime rated set, 200 kW/ 275 KVA = 0.727272.
Your comment is welcome David. Scotty and I often discover discrepancies between the characteristics of small sets versus larger sets. That is why I posted the question.
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Turbo Alternator Rotor bars arcing
Big sets don't normally have a declared overload margin in the manner that smaller units do. The machine capability is dependant on external factors such as ambient and coolant temperature, but it is normally stated as a single rating at specified conditions of temperature, PF, etc. Normally machine protection is set to trip the unit a little below the thermal rating of the insulation system. This is to ensure the longevity of the insulation because many customers will specify a minimum service life of (say) 25 years.
David is right - the capability diagram does pull in quite a bit from the stator current limit, primarily due to the difficulty of extracting heat from the rotor which imposes a limit on rotor current, and thus a limit on reactive power export. The heat removal problem gets worse as units get bigger, and while this isn't a large generator by utility standards it is still quite a big machine.
data74,
Please heed David's advice on setting machine protection. That's an expensive machine, even if it is an old one, and you really need to get external help. Some of those protection functions are tricky to set even for someone with experience. I imagine Brush probably issued a relay setting sheet when the machine was built. Brush do have a pretty good archive at Loughborough so they might be able to help. Are you trying to replace an old electro-mechanical relay scheme by any chance?
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If we learn from our mistakes I'm getting a great education!
RE: Turbo Alternator Rotor bars arcing
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Turbo Alternator Rotor bars arcing